1. Field of the Invention
The present invention relates to a carrier frequency error detector in a receiver which receives and demodulates broadcasting using an OFDM system, and a detection method thereof.
2. Description of the Related Art
As a modulation system for terrestrial wave digital broadcasting, OFDM (Orthogonal Frequency Division Multiplex) is known, and this system is widely used in terrestrial wave digital broadcasting, including the DVB-T (Digital Video Broadcasting-Terrestrial) standard in Europe and the ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) standard in Japan. OFDM uses many carriers which are set at equal intervals on a frequency axis, that is, it uses a multi-carrier system, and is a modulation system where the OFDM symbols (hereafter referred to as “symbols”), acquired by modulating these carriers based on the transmission data, are sequentially sent at each unit transmission time (hereafter referred to as “symbol transmission period”).
In the case of terrestrial wave digital broadcasting using the OFDM system, pilot carrier signals, to make it easier to estimate the transfer characteristics of the transmission path, are used along with the data carrier signals for transmitting information data, such as images and voice. For example, in such a standard as the above mentioned ISDB-T and DVB-T, a pilot carrier signal referred to as a scattered pilot (SP) signal (hereafter referred to as “SP signal”) is specified. When an OFDM symbol space comprised of two dimensions of a carrier frequency and symbol time is assumed, it is known that the SP signals are superimposed on a specific position in this space, and the complex amplitude thereof, that is the absolute amplitude and the phase of the SP signal, is also predetermined. Therefore a receiver, which receives digital broadcasting based on these standards, can estimate the transmission characteristics of a radio wave propagation path of each carrier using the SP signal, and can perform correction processing and equalization processing on the receive signals based on this estimation result. Carriers other than the SP signals included in the OFDM symbol space are, needless to say, data carrier signals on which modulation, such as QAM and PSK, has been performed.
When an OFDM signal is observed in a time domain, the symbol is comprised of a guard period and a valid symbol period. The signal included in the valid symbol period is used for decoding the receive data included in the OFDM signal. The guard period is a part created to prevent interference by delay signals due to multi-paths, and normally a part of the signal in the rear part of the valid symbol period is cyclically copied and used.
When the OFDM signal is decoded, the interference between carriers, due to the frequency deviation of carrier waves, is easily generated since the frequency space between each carrier is narrow, so high precision frequency synchronization is required. For the frequency synchronization, the carrier frequency deviation detector and the detection method shown, for example, in Television Society Technical Report, Vol. 20, No. 53, pp. 61-66 (Oct. 17, 1996), “Study of Frequency Synchronization of OFDM Modulation” (Tomohiro Kimura, Kenichiro Hayashi, Sadashi Kageyama, and three others), are used in a conventional OFDM receiver.
The configuration and operation of a conventional carrier frequency deviation detector will be described in brief. As mentioned above, in the case of the OFDM signal, the signal at the rear part of the valid symbol period is copied as the signal in the guard period, so the signal in the guard period and the signal at the rear part of the valid symbol period are essentially the same. However if deviation occurs to the carrier frequency, phase rotation occurs between the signals of these two period. In a conventional system, the correlation vector of the signals of these two periods is determined, and the carrier frequency deviation is estimated from the value of the phase rotation. In this system, the estimated frequency deviation of the carrier is determined by estimating the deviation from the correlation value between the complex base area series signal, which is the output of the front end of the receiver, and the signal acquired by delaying this signal for a valid symbol length and determining the complex conjugate value thereof.
In a conventional system, however, if such a disturbance as multi-path interference occurs, the estimation accuracy of the carrier frequency deviation deteriorates because of the influence of the correlation which is generated on signals in a portion other than the guard period. Therefore the residual frequency deviation in the complex base area serial signal of the entire OFDM receiver increases, and the error rate characteristic of the receive signal deteriorates by ICI (Inter-Carrier Interference), which is generated between carriers.